Limit regression test workers to 1

.github/workflows/tests.yml

@@ -109,15 +109,19 @@ jobs:
   regression-tests:
     name: Run mypy on the test cases
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        platform: ["linux", "darwin", "win32"]
+      fail-fast: false
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v4
         with:
-          python-version: "3.11"
+          python-version: "3.12"
           cache: pip
           cache-dependency-path: requirements-tests.txt
       - run: pip install -r requirements-tests.txt
-      - run: python ./tests/regr_test.py --all --verbosity QUIET
+      - run: python ./tests/regr_test.py --platform=${{ matrix.platform }} --all-python-versions --verbosity QUIET
 
   pyright:
     name: Test typeshed with pyright

tests/regr_test.py

@@ -82,6 +82,14 @@ class Verbosity(IntEnum):
         "Note that this cannot be specified if --platform and/or --python-version are specified."
     ),
 )
+parser.add_argument(
+    "--all-python-versions",
+    action="store_true",
+    help=(
+        'Run tests on all available Python versions (defaults to "False"). '
+        "Note that this cannot be specified if --all and/or --python-version are specified."
+    ),
+)
 parser.add_argument(
     "--verbosity",
     choices=[member.name for member in Verbosity],
@@ -308,7 +316,11 @@ def concurrently_run_testcases(
     printer_thread = threading.Thread(target=print_queued_messages, args=(event,))
     printer_thread.start()
 
-    with concurrent.futures.ThreadPoolExecutor(max_workers=os.cpu_count()) as executor:
+    # Limit the number of workers to 1 for now. There are intermittent mypy
+    # crashes when running with multiple workers. See
+    # https://github.com/python/typeshed/issues/11220 for details.
+    max_workers = 1  # os.cpu_count() or 2
+    with concurrent.futures.ThreadPoolExecutor(max_workers=max_workers) as executor:
         # Each temporary directory may be used by multiple processes concurrently during the next step;
         # must make sure that they're all setup correctly before starting the next step,
         # in order to avoid race conditions
@@ -334,9 +346,16 @@ def main() -> ReturnCode:
     if args.all:
         if args.platforms_to_test:
             parser.error("Cannot specify both --platform and --all")
+        if args.all_python_versions:
+            parser.error("Cannot specify both --all-python-versions and --all")
         if args.versions_to_test:
             parser.error("Cannot specify both --python-version and --all")
         platforms_to_test, versions_to_test = SUPPORTED_PLATFORMS, SUPPORTED_VERSIONS
+    elif args.all_python_versions:
+        if args.versions_to_test:
+            parser.error("Cannot specify both --python-version and --all-python-versions")
+        platforms_to_test = args.platforms_to_test or [sys.platform]
+        versions_to_test = SUPPORTED_VERSIONS
     else:
         platforms_to_test = args.platforms_to_test or [sys.platform]
         versions_to_test = args.versions_to_test or [PYTHON_VERSION]